The present invention relates generally to a process of coating a nuclear fuel with a burnable absorber, also referred to as a burnable poison, and more particularly, to an improved process of coating a nuclear fuel using chemical vapor deposition techniques with boron nitride material acting as a burnable absorber.
It is known that nuclear fuel may have various shapes such as plates, columns, and even fuel pellets disposed in end-to-end abutment within a tube or cladding made of zirconium alloy or stainless steel. The fuel pellets contain fissionable material, such as uranium dioxide, thorium dioxide, plutonium dioxide, or mixtures thereof. The fuel rods are usually grouped together to form a fuel assembly. The fuel assemblies are arranged together to constitute the core of a nuclear reactor.
It is well known that the process of nuclear fission involves the disintegration of the fissionable nuclear fuel material in the nuclear core into two or more fission products of lower mass number. Among other things, the process also includes a net increase in the number of available free neutrons which are the basis for a self-sustaining reaction. When a reactor has operated over a period of time, the fuel assembly with fissionable material must ultimately be replaced due to depletion. Inasmuch as the process of replacement is time consuming and costly, it is desirable to extend the life of a given fuel assembly as long as practically feasible. For that reason, deliberate additions to the reactor fuel of parasitic neutron-capturing elements in calculated small amounts may lead to highly beneficial effects on a thermal reactor. Such neutron-capturing elements are usually designated as "burnable absorbers" if they have a high probability or cross-section for absorbing neutrons while producing no new or additional neutrons or changing into new absorbers as a result of neutron absorption. During reactor operation, the burnable absorbers are progressively reduced in amount so that there is a compensation made with respect to the concomitant reduction in the fissionable material.
The life of a fuel assembly may be extended by combining an initially larger amount of fissionable material, as well as a calculated amount of burnable absorber. During the early stages of operation of such a fuel assembly, excessive neutrons are absorbed by the burnable absorber which undergoes transformation to elements of low neutron cross-section which do not substantially affect the reactivity of the fuel assembly in the latter period of its life, when the availability of fissionable material is lower. The burnable absorber compensates for the larger amount of fissionable material during the early life of the fuel assembly, but progressively less absorber captures neutrons during the latter life of the fuel assembly, so that a long life at relatively constant fission level is assured for the fuel assembly. Accordingly, with a fuel assembly containing both fuel and burnable absorber in carefully proportioned quantity, an extended fuel assembly life can be achieved with relatively constant neutron production and reactivity. Burnable absorbers which may be used include boron, gadolinium, samarium, europium, and the like, which upon the absorption of neutrons result in isotopes of sufficiently low neutron capture cross-section so as to be substantially transparent to neutrons.
The incorporation of burnable absorbers in fuel assemblies has thus been recognized in the nuclear field as an effective means of increasing fissionable material capacity and thereby extending reactor core life. Burnable absorbers are used either uniformly mixed with the fuel, i.e., distributed absorber, are coated onto the nuclear fuel or are placed discretely as separate elements in the reactor core. Thus the net reactivity of the core is maintained relatively constant over the active life of the core.
Boron nitride coatings have been found to be the most desirable coatings for fuel pellets, based upon the coatings' adherency and boron nitride's expansion coefficient being closest to that of uranium dioxide; however, attempts to coat a fuel pellet with boron nitride have been quite unsuccessful. Techniques such as sputtering, ion plating, plasma spraying, and chemical vapor deposition have been attempted. It has been found that by using the techniques of sputtering or ion plating, although a coating can be achieved, the rate of deposition of boron nitride is extremely slow. The extremely slow deposition rate results in such processes being extremely expensive and commercially undesirable, since each of the processes require high voltage power supplies and high vacuum chambers. With respect to plasma spraying of a boron nitride coating on a uranium dioxide pellet, it has been found that control of deposition of the coating has been extremely difficult. Additionally, chemical bond splitting of the boron and nitrogen atom has been observed at the temperatures required for plasma spraying, resulting in the stoichiometric ratio of coating being unbalanced in that the uranium dioxide pellet is coated with generally boron alone. Finally, attempts at chemical vapor depositing a boron nitride coating on a uranium dioxide fuel pellet have proven to be unsuccessful since the reaction between boron trichloride (BCl.sub.3) and ammonia (NH.sub.3) requires temperatures greater than 1000.degree. C. At such high temperatures the HCL by-product produced during the reaction between BCl.sub.3 and NH.sub.3 causes severe problems in that the uranium dioxide fuel pellet decomposes due to the corrosive action of the HCL on the UO.sub.2 pellet at these temperatures.
There is disclosed a method for coating of elemental boron in U.S. Pat. No. 3,427,222, wherein a uranium dioxide fuel pellet substrate is coated with the burnable absorber boron applied by chemical vapor deposition at a temperature greater than 800.degree. C. The coating is achieved by the thermal reduction of boron trichloride in the presence of hydrogen. It is noted that the deposition rate at temperatures below 800.degree. C. is relatively slow, whereas a boron coating formed at process temperatures above 1000.degree. C. is not as adherent.
Accordingly, it can be appreciated that there is an unsolved need for a method for applying a boron nitride coating to be used as as burnable absorber into a nuclear fuel pellet which is commercially desirable, relatively inexpensive, and not harmful to the fuel pellet itself.